Lantronix
SM12XPA Web User Guide
33848 Rev. A
Page
197
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Frames sent between the supplicant and the switch are special 802.1X frames, known as EAPOL (EAP
Over LANs) frames. EAPOL frames encapsulate EAP PDUs (RFC3748). Frames sent between the switch
and the RADIUS server are RADIUS packets. RADIUS packets also encapsulate EAP PDUs together with
other attributes like the switch's IP address, name, and the supplicant's port number on the switch. EAP
is very flexible, in that it allows for different authentication methods, like MD5-Challenge, PEAP, and TLS.
The important thing is that the authenticator (the switch) doesn't need to know which authentication
method the supplicant and the authentication server are using, or how many information exchange
frames are needed for a particular method. The switch simply encapsulates the EAP part of the frame
into the relevant type (EAPOL or RADIUS) and forwards it.
When authentication is complete, the RADIUS server sends a special packet containing a success or
failure indication. Besides forwarding this decision to the supplicant, the switch uses it to open up or
block traffic on the switch port connected to the supplicant.
Note
: Suppose two backend servers are enabled and that the server timeout is configured to X seconds (using
the AAA configuration page) and suppose that the first server in the list is currently down (but not considered
dead). Now, if the supplicant retransmits EAPOL Start frames at a rate faster than X seconds, then it will never
get authenticated, because the switch will cancel on-going backend authentication server requests whenever it
receives a new EAPOL Start frame from the supplicant. And since the server hasn't yet failed (because the X
seconds haven't expired), the same server will be contacted upon the next backend authentication server
request from the switch. This scenario will loop forever. Therefore, the server timeout should be smaller than
the supplicant's EAPOL Start frame retransmission rate.
Single 802.1X
: In port-based 802.1X authentication, once a supplicant is successfully authenticated on a
port, the whole port is opened for network traffic. This allows other clients connected to the port (for
instance through a hub) to piggy-back on the successfully authenticated client and get network access
even though they really aren't authenticated. To overcome this security breach, use the Single 802.1X
variant. Single 802.1X is really not an IEEE standard, but features many of the same characteristics as
does port-based 802.1X. In Single 802.1X, at most one supplicant can get authenticated on the port at a
time. Normal EAPOL frames are used in the communication between the supplicant and the switch. If
more than one supplicant is connected to a port, the one that comes first when the port's link comes up
will be the first one considered. If that supplicant doesn't provide valid credentials within a certain
amount of time, another supplicant will get a chance. Once a supplicant is successfully authenticated,
only that supplicant will be allowed access. This is the most secure of all the supported modes. In this
mode, the Port Security module is used to secure a supplicant's MAC address once successfully
authenticated.
Multi 802.1X
: In port-based 802.1X authentication, once a supplicant is successfully authenticated on a
port, the whole port is opened for network traffic. This allows other clients connected to the port (for
instance through a hub) to piggy-back on the successfully authenticated client and get network access
even though they really aren't authenticated. To overcome this security breach, use the Multi 802.1X
variant.
Multi 802.1X is really not an IEEE standard, but features many of the same characteristics as does port-
based 802.1X. Multi 802.1X is - like Single 802.1X - not an IEEE standard, but a variant that features
many of the same characteristics. In Multi 802.1X, one or more supplicants can get authenticated on the
same port at the same time. Each supplicant is authenticated individually and secured in the MAC table
using the Port Security module.
In Multi 802.1X it is not possible to use the multicast BPDU MAC address as destination MAC address for
EAPOL frames sent from the switch towards the supplicant, since that would cause all supplicants
attached to the port to reply to requests sent from the switch. Instead, the switch uses the supplicant's
